It is well known that phyllosilicates, such as smectite clays, e.g., sodium montmorillonite and calcium montmorillonite, can be treated with organic molecules, such as organic ammonium ions, to intercalate the organic molecules between adjacent, planar silicate layers, thereby substantially increasing the interlayer (interlaminar) spacing between the adjacent silicate layers. The thustreated, intercalated phyllosilicates, then can be exfoliated, e.g., the silicate layers are separated, e.g., mechanically, by high shear mixing. The individual silicate layers, when admixed with a matrix polymer, before, after or during the polymerization of the matrix polymer, e.g., a polyamide--see 4,739,007; 4,810,734; and 5,385,776--have been found to substantially improve one or more properties of the polymer, such as mechanical strength and/or high temperature characteristics.
Exemplary of such prior art composites, also called "nanocomposites", are disclosed in published PCT disclosure of Allied Signal, Inc. WO 93/04118 and U.S. Pat. No. 5,385,776, disclosing the admixture of individual platelet particles derived from intercalated layered silicate materials, with a polymer to form a polymer matrix having one or more properties of the matrix polymer improved by the addition of the exfoliated intercalate. As disclosed in WO 93/04118, the intercalate is formed (the interlayer spacing between adjacent silicate platelets is increased) by adsorption of a silane coupling agent or an onium cation, such as a quaternary ammonium compound, having a reactive group which is compatible with the matrix polymer. Such quaternary ammonium cations are well known to convert a highly hydrophilic clay, such as sodium or calcium montmorillonite, into an organophilic clay capable of sorbing organic molecules. A publication that discloses direct intercalation (without solvent) of polystyrene and poly(ethylene oxide) in organically modified silicates is Synthesis and Properties of Two-Dimensional Nanostructures by Direct Intercalation of Polymer Melts in Layered Silicates, Richard A. Vaia, et al., Chem. Mater., 5:1694-1696(1993). Also as disclosed in Adv. Materials, 7, No. 2: (1985), pp, 154-156, New Polymer Electrolyte Nanocomposites: Melt Intercalation of Poly(Ethylene Oxide) in Mica-Type Silicates, Richard A. Vaia, et al., poly(ethylene oxide) can be intercalated directly into Na-montmorillonite and Li-montmorillonite by heating to 80.degree. C. for 2-6 hours to achieve a d-spacing of 17.7 .ANG.. The intercalation is accompanied by displacing water molecules, disposed between the clay platelets with polymer molecules. Apparently, however, the intercalated material could not be exfoliated and was tested in pellet form. It was quite surprising to one of the authors of these articles that exfoliated material could be manufactured in accordance with the present invention.
Previous attempts have been made to intercalate polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVOH) and poly(ethylene oxide) (PEO) between montmorillonite clay platelets with little success. As described in Levy, et al., Interlayer Adsorption of Polyvinylpyrrolidone on Montmorillonite, Journal of Colloid and Interface Science, Vol. 50, No. 3, March 1975, pages 442-450, attempts were made to sorb PVP (40,000 average M.W.) between monoionic montmorillonite clay platelets (Na, K, Ca and Mg) by successive washes with absolute ethanol, and then attempting to sorb the PVP by contact with 1% PVP/ethanol/water solutions, with varying amounts of water, via replacing the ethanol solvent molecules that were sorbed in washing (to expand the platelets to about 17.7 .ANG.). Only the sodium montmorillonite had expanded beyond a 20 .ANG. basal spacing (e.g., 26 .ANG. and 32 .ANG.), at 5.sup.+ % H.sub.2 O, after contact with the PVP/ethanol/H.sub.2 O solution. It was concluded that the ethanol was needed to initially increase the basal spacing for later sorption of PVP, and that water did not directly affect the sorption of PVP between the clay platelets (Table II, page 445), except for sodium montmorillonite. The sorption was time consuming and difficult and met with little success.
Further, as described in Greenland, Adsorption of Polyvinyl Alcohols by Montmorillonite, Journal of Colloid Sciences, Vol. 18, pages 647-664 (1963), polyvinyl alcohols containing 12% residual acetyl groups could increase the basal spacing by only about 10 .ANG. due to the sorbed polyvinyl alcohol (PVOH). As the concentration of polymer in the intercalant polymer-containing solution was increased from 0.25% to 4%, the amount of polymer sorbed was substantially reduced, indicating that sorption might only be effective at polymer concentrations in the intercalant polymer-containing composition on the order of 1% by weight polymer, or less. Such a dilute process for intercalation of polymer into layered materials would be exceptionally costly in drying the intercalated layered materials for separation of intercalate from the polymer carrier, e.g., water, and, therefore, apparently no further work was accomplished toward commercialization.
In accordance with an important feature of the present invention, intercalation is achieved using a water-soluble or water-insoluble (organic solvent-soluble) monomer, oligomer (herein defined as a pre-polymer having 2 to about 15 recurring monomeric units, which can be the same or different) or polymer (herein defined as having more than about 15 recurring monomeric units, which can be the same or different) composition for intercalation having at least about 2%, preferably at least about 5% by weight, more preferably at least about 10% by weight intercalant monomer, intercalant oligomer, or intercalant polymer concentration, most preferably about 30% to about 80% by weight monomer, oligomer and/or polymer, based on the weight of monomer, oligomer and/or polymer and carrier (e.g., water with or without another solvent for the intercalant monomer, intercalant oligomer, or intercalant polymer) to achieve better sorption of the intercalant between phyllosilicate platelets. Regardless of the concentration of intercalant in liquid solvent of the intercalating composition, the intercalating composition should have an intercalant layered material ratio of at least 1:20, preferably at least 1:10, more preferably at least 1:5, and most preferably about 1:4 to achieve efficient intercalation of the intercalant between adjacent platelets of the layered material. The intercalant (monomer, oligomer and/or polymer) sorbed between and permanently bonded to the silicate platelets causes separation or added spacing between adjacent silicate platelets and, for simplicity of description, the monomers, oligomers and polymers are hereinafter called the "intercalant" or "intercalant monomer", or "monomer intercalant", or "intercalant polymer" or "polymer intercalant". In this manner, the monomers, oligomers and/or polymers will be sorbed sufficiently to increase the interlayer spacing of the phyllosilicate in the range of about 5 .ANG. to about 100 .ANG., preferably at least about 10 .ANG., for easier and more complete exfoliation, in a commercially viable process, regardless of the particular phyllosilicate or intercalant polymer.
A phyllosilicate, such as a smectite clay, can be intercalated sufficiently for subsequent exfoliation by sorption of monomers, polymers or oligomers that have carbonyl, hydroxyl, carboxyl, amine, amide, ether, ester, sulfate, sulfonate, sulfinate, sulfamate, phosphate, phosphonate, phosphinate functionalities, or aromatic rings to provide metal cation complexing between two functional groups of one or two intercalant molecules and the metal cations complexing to the inner surfaces of the phyllosilicate platelets. Sorption and metal cation electrostatic attraction or bonding of a platelet metal cation between two oxygen or nitrogen atoms of the molecules; or the electrostatic bonding between the interlayer cations in hexagonal or pseudohexagonal rings of the smectite layers and an intercalant aromatic ring structure increases the interlayer spacing between adjacent silicate platelets or other layered material to least about 5 .ANG., preferably at least about 10 .ANG., and more preferably to at least about 20 .ANG., and most preferably to an interlayer spacing in the range of about 30 .ANG. to about 45 .ANG.. Such intercalated phyllosilicates can be exfoliated into individual phyllosilicate platelets before or during admixture with a liquid carrier or solvent, for example, one or more monohydric alcohols, such as methanol, ethanol, propanol, and/or butanol; polyhydric alcohols, such as glycerols and glycols, e.g., ethylene glycol, propylene glycol, butylene glycol, glycerine and mixtures thereof; aldehydes, ketones, carboxylic acids; amines; amides; and other solvents, for delivery of the solvent in a thixotropic composition, or for delivery of any active hydrophobic or hydrophilic organic compound, such as a topically active pharmaceutical, dissolved or dispersed in the carrier or solvent, in a thixotropic composition.
In accordance with an important feature of the present invention, it has been found that the addition of metal cations, preferably during intercalation and/or exfoliation, or the addition of metal cations to a nanocomposite composition of an organic liquid and an intercalate or exfoliate thereof, unexpectedly increases the viscosity of an organic liquid-containing nanocomposite composition. It is preferred that the metal cation has a valence of at least 2, more preferably at least 3, although monovalent salts (preferably not NaOH) also increase the viscosity to a lesser degree. The anion portion of the cation-containing compound, added to provide cations, may be inorganic or organic and the cation-containing compound is added in solution (with water and/or an organic solvent) to provide metal cations, as well as anions, in solution. The addition of the metal cations in solution to the intercalating composition results in sufficient intercalation for easy exfoliation using less intercalant. It is theorized that polar moieties from the intercalant molecules, which complex to the interlayer cations in the interlayer spaces between the platelets of the layered material, also complex with the added cations, and the complexed metal salt-derived cations carry their dissociated anions along with the cations, in the interlayer space, in order to maintain charge neutrality within the interlayer spaces of the layered material. It is theorized that such double intercalant complexing (intercalant with interlayer cations and with cations from the added metal salt compound) occurs on adjacent, opposed platelet surfaces, resulting in repulsion between closely spaced dissociated anions carried by the added cations, resulting in increased basal spacing and more complete exfoliation using less intercalant.
Addition of the dissolved salt compounds after exfoliation also increases the viscosity of the organic liquid/exfoliate nanocomposite composition since the added cations provide increased and essentially total exfoliation of tactoids so that more individual platelets are available for viscosity increase.
Depending upon the conditions that the composition is subjected to during intercalation and exfoliation, particularly temperature; pH; and amount of water and/or organic liquid contained in the intercalating composition, the intercalate and/or exfoliate/carrier composition can be formed to any desired viscosity, e.g., at least about 100 centipoises, preferably at least about 500-1000 centipoises, whether or not gelled, and particularly to extremely high viscosities of about 5,000 to about 5,000,000 centipoises. The compositions are thixotropic so that shearing will lower viscosity for easier delivery, and then by reducing shear or eliminating shear, the compositions will increase in viscosity. The intercalant intercalates between the spaces of adjacent platelets of the layered material for easy exfoliation, and complexes with the metal cations on the platelet surfaces where the intercalant remains after the intercalant, or exfoliate thereof, is combined with the carrier/solvent. It is theorized that the intercalant coating on the surfaces of the clay platelets is ionically complexed with interlayer cations, as well as with the added, metal salt-derived cations, and participates (aids) in the viscosification and thixotropy of the exfoliate/solvent composition. However, other forms of bonding such as electrostatic complexing, chelation, dipole/dipole, hydrogen bonding and/or Van Der Waals forces or molecular complexing also may be responsible for the adherence of the intercalant to the surfaces of the layered material, either entirely, or in part.